32 
Psyche 
[March 
otherwise employ. This is not to suggest that barrier webs are im- 
penetrable; they are easily penetrated, but in the process the occupant 
is forewarned. 
Barrier webs do not act as prey catching structures per se though 
they may slow down grasshoppers and other relatively strong prey 
and thereby facilitate capture. Twenty prey items (leaf-hoppers and 
flies) were tossed onto the primary barrier webs of 20 different A. 
trifasciata. In 19 of 20 cases the prey were either ignored or the 
spider plucked strands of the orb with no subsequent action. In one 
instance the prey, a leafhopper, struggled to a point very close to the 
margin of the tangle and the orb web where it was successfully 
attacked. In the capture the spider did not completely leave the orb 
web at any time. Incidental field observations during the summer 
are in close agreement with the results of this test. 
The occurrence of barrier webs changes markedly with the instar 
of A. trifasciata (Table 1). With the increase in secondary barrier 
webs, the primary webs are made less cone-like and the mesh pro- 
gressively coarser. Adult males rarely build complete webs since they 
generally abandon the web building habit upon reaching maturity and 
begin the search for females. When the male locates a female or 
penultimate female web site, he remains on the barrier web until 
mating can take place. Mating occurs generally within a few days, 
sometimes within a few hours after the female adult molt. Female 
TABLE 1. 
Barrier webs and stabilimenta of all field instars of A. trifasciata 
Instar 
Number 
of 
webs 1 
Primary 
barrier 
web 
Secondary 
barrier 
web 
Stabilimenta : 
mean number 
bands/web ± S.E. 
2 
300 
300 
0 
0.00 ± 0.000 
3 
200 
200 
0 
0.00 ± 0.000 
4 
36 
36 
0 
0.11 ± 0.053 
5 
20 
20 
0 
0.25 ± 0.104 
6 
34 
34 
1 
0.38 ± 0.095 
Juvenile $ 
7 
39 
39 
17 
1.02 ± 0.107 
Penultimate $ 
7 
102 
102 
41 
0.76 ± 0.064 
Juvenile $ 
8 
95 
95 
48 
1.40 ± 0.063 
Penultimate $ 
9 
39 
39 
27 
1.72 ± 0.097 
Adult $ 
10 
29 
1 
0 
0.38 ± 0.126 
894 
866 
134 
Represents 
minimum 
number of 
webs examined with 
no more than one 
web/spider counted. 
